Northrop P-61 Black Widow

Introduction

The Northrop P-61 Black Widow was the largest and heaviest fighter aircraft to enter service with the USAAF during the Second World War. It was also the first American aircraft specifically designed from the outset for the night fighting role. It made its operational debut on the Russian Front in the summer of 1944 and was the standard USAAF night fighter until it was replaced by the Grumman P-65 in mid-1945. However, as it was replaced in the night-fighter role, it found its true calling as a night intruder and ground attack aircraft. As a result, the Black Widow did not chalk up a particularly impressive number of aircraft kills but became responsible for a long litany of destroyed trains, shot-up vehicle parks and savaged airfields.

Early Development

The saga of the Northrop P-61 Black Widow began when, following the Halifax-Butler Coup, a steady stream of British scientists and engineers started quietly leaving the U.K. Amongst these were British scientists and engineers who had been working on the early versions of AI (Airborne Interception) radar sets which could be carried aboard airplanes, enabling them to detect and intercept other airplanes in flight without having to rely on ground installations. As a result of these meetings, a preliminary specification was drawn up and passed on to Air Technical Service Command at Wright Field in late 1940. Because of the heavy weight of the early AI radar and because of the high loiter time required, a twin-engined aircraft was envisaged.

Northrop's proposal was a twin-engined monoplane powered by a pair of Pratt & Whitney R-2800 Double Wasp air-cooled radial engines mounted in low-slung nacelles underneath the wings. The nacelles tapered back into twin tail booms which were connected to each other by a large horizontal stabilizer and elevator. The long fuselage housed a crew of three. The crew consisted of a pilot, a gunner for the nose turret, and a radar operator/rear turret gunner. Each turret housed four 0.50-inch machine guns. A tricycle landing gear was fitted. Estimated weights were 16,245 pounds empty, 22,654 pounds gross. Height was 13 feet 2 inches, length was 45 feet 6 inches, and wingspan was 66 feet. These dimensions and weights were more typical of a bomber than a fighter. Following evaluation, Northrop then presented a revised design to the USAAC. An additional gunner's station was fitted. Nose and tail turrets of the original version were replaced by twin 0.50-in machine guns in the belly, and four 0.50-in machine guns in a dorsal turret. The crew was now up to four—a pilot, a radar operator, and two gunners. The airborne intercept radar was moved to the nose.

The design was revised still further with the belly turret was deleted, and the crew was changed back to three—pilot, gunner, and radar operator. The pilot sat up front, and the gunner sat immediately behind and above the pilot. The gunner was to operate the turret via remote control, using a special sight attached to a swiveling chair. A "stepped-up" canopy was used to provide a clear field of view for the gunner. The rear fuselage with its clear tail cone provided the radar operator with an excellent rearward view which enabled him to act as a tail gunner if the plane happened to be attacked from astern. Optionally, the dorsal turret guns could be "locked" into the forward-pointing position, so that they could be fired by the pilot. The belly guns were deleted, and four 20-mm cannon were to be fitted in the wings.

The Army was generally pleased with the design, but they suggested some changes. The mockup was ready for inspection in April of 1941. At that time, it was decided to move the four 20-mm cannon from the outboard portion of the wings to the belly. This was done to improve the ease of maintenance and to make the airflow over the wing smoother. The internal fuel capacity was increased from 540 gallons in two tanks to 646 gallons in four self-sealed tanks built into the wings. A letter of intent was initiated on December 24, 1941, which called for 100 P-61 production aircraft and spares. Fifty more were ordered on January 17, 1942. The order was increased to 410 aircraft on February 12, 1942.

The XP-61 flew at Northrop Field for the first time on May 26, 1942, powered by a pair of Pratt & Whitney R-2800-10 radials of 2000 hp each. In keeping with its nocturnal role, it was finished in black overall, befitting its popular name that was taken from the poisonous North American spider. Wingspan was 66 feet, length was 48 feet 10 inches, and height was 14 feet 2 inches. Weights were 19,245 pounds empty, 25,150 pounds gross, and 28,870 pounds maximum. The aircraft was equipped with only a mockup of the top turret, as General Electric had not yet been able to deliver the real thing because of the higher priority of other projects.

The XP-61 had a maximum speed of 370 mph at 29,900 feet, and an altitude of 20,000 feet could be attained in 9 minutes. Service ceiling was 33,100 feet, and maximum range was 1450 miles.

Thirteen YP-61s were delivered during August and September of 1943. In order to reduce vibrations from firing the 0.50-inch turret machine guns, some YP-61s were fitted with only two turret guns. The assignments of the YP-61s were varied. Some stayed at Northrop for flight testing and factory training of maintenance personnel. Some went to Wright Field in Ohio for service testing. Others went to Florida where they underwent operational suitability testing. The YP-61s initially did not have any airborne interception radar fitted, but the SCR-520, a preproduction version of the SCR-720 which was to go into the production P-61A, was installed.

Variants

Northrop P-61A Black Widow

The P-61A was the first production version of the Black Widow. The first P-61A-1-NO rolled off the production line in October 1943. It differed from the experimental and service-test aircraft in having a stronger framework structure for the pilot's, radar observer's and gunner's canopies. Tests with the YP-61 had uncovered the fact that the greenhouse and tail cone were so weak that they could actually implode under the pressure built up during high-speed dives. This strengthening eliminated the smooth flow of the greenhouse and created a sharper and more abrupt change in angle in that area. The welded magnesium alloy booms of the prototypes were replaced by more conventional aluminum alloy booms, since they were less expensive and easier to manufacture.

Only the first thirty-seven of the 45 P-61A-1s were actually equipped with the dorsal turrets. In fact, more than half of all P-61As built actually had this turret deleted. The primary reason was the occurrence of severe aerodynamic buffeting when the turret was being either elevated or rotated in azimuth during flight. Many flight-test hours were spent in trying to solve this problem, but it was never completely eliminated. In fact, this problem was often so severe that many P-61As in the field had the four 0.50-inch machine guns in the top turret permanently locked into the forward-firing position, being fired only by the pilot, with the gunner having no control at all. In many cases, the top turret was completely removed from the aircraft, and the cavity left behind by the deletion of the gun turret was filled up by an extra fuel tank and was faired over.

Most of the P-61A-1-NOs went to the USAAF night fighter squadrons over the Russian Front. The 6th Night Fighter Squadron was the first to receive the new fighter. The P-61 was quite docile despite its size. Full control of the aircraft could be maintained with one engine out, even when fully loaded. The plane could be slow-rolled into a dead engine, a maneuver which would ordinarily have been suicidal.

Specifications of the P-61A Black Widow

The P-61A-5-NO was powered by a pair of 2250 hp R-2800-65 engines. Maximum speed was 322 mph at sea level, 355 mph at 10,000 feet, and 369 mph at 20,000 feet. Range (clean) was 415 miles at 319 mph at 20,000 feet and 1010 miles at 224 mph at 10,000 feet. Range with maximum external fuel was 1900 miles at 221 mph at 10,000 feet. An altitude of 5000 feet could be reached in 2.2 minutes, and 15,000 feet in 7.6 minutes. Service ceiling was 33,100 feet. Weights were 20,965 pounds empty, 27,600 pounds normal loaded, and 32,400 pounds maximum. Dimensions were wingspan 66 feet 0 inches, length 48 feet 11 inches, height 14 feet 2 inches, and wing area 664 square feet. Armament was 4 20mm cannon in the belly and four 0.5 inch machine guns in the dorsal turret. After the first 100 aircraft, a pylon on the outer wing was installed to carry either a pair of 265 gallon fuel tanks (later 310 gallon tanks were fitted) or a pair of 1600-pound bombs.

Northrop P-61B Black Widow

The P-61B was the next production version of the Black Widow. It was basically similar to the P-61A version, but introduced numerous improvements and refinements that were suggested by operational experience in the field. The P-61B version of the Black Widow introduced the improved SCR-720C A/I radar. The P-61B also had an eight-inch longer crew nacelle. The A-model's hydraulically-operated main landing-gear doors which had experienced reliability problems in the field were replaced by mechanically-operated doors. The P-61B introduced split main landing gear doors. The split main-gear doors allowed the aft three-quarters of the doors to close back down again after the gear had been extended, preventing mud, rocks and other debris from being thrown up into the wheel wells during takeoffs or landings. A main landing gear down-lock emergency release was introduced, which allowed the pilot to release the locks in an emergency even if the entire hydraulic system malfunctioned. A safety latch was added to the main gear hydraulic valve handle to eliminate the possibility of the pilot inadvertently retracting the gear while the plane was on the ground. The B-model had a bigger and better heater system for the crew, and it had automatically- operated lower engine cowl flops, oil-cooler air exit flaps, and intercooler flaps. The oil tanks were mounted inside the engine nacelles instead of inside the outer wings. A taxi lamp was added to the landing gear strut. The aileron trim tabs were deleted, and a built-in fire extinguisher system was added.

The turret buffeting problems having been solved, all P-61Bs carried the full eight-gun armament.

Northrop P-61C Black Widow

The next version of the Black Widow to enter service was the P-61C, which was a high-performance variant designed to rectify some of the combat deficiencies encountered with the A and B variants. The Black Widow was just not fast enough to make it a really great night fighter. In addition, operational ceiling could stand for some improvement. By mid-1943, even before the combat debut of the Black Widow, both the USAAF and Northrop had come to the same conclusion, and on November 11, 1943, Northrop was given the go-ahead to proceed with an improved Black Widow, the XP-61C. This was essentially the same Black Widow airframe but powered by turbosupercharged R-2800-73 radials offering a war emergency power of 2800 hp.

The XP-61C program lead to a scandal and the first substantive break between Northrop and the USAF. Jack Northrop was obsessed with flying wing aircraft and, in particular, with promoting his XB-35 flying wing bomber as a replacement for the B-36. He was responsible for diverting much effort away from the XP-61C into the XB-35 program, seriously delaying the former. This eventually resulted in an explosive meeting during which Northrop was instructed to cease all work on the XB-35 and concentrate on the XP-61C. Northrop responded by threatening to “go public” on the XB-35 vs B-36 debate, alleging that the selection of the B-36 was the result of a corrupt conspiracy against his company. This would, of course, have compromised the secrecy that surrounded B-36 development. USAF action was swift and decisive. Northrop was told to continue with the XB-35 (the Air Force being determined that it would never proceed beyond a single prototype but the XP-61C program was taken away from Northrop and assigned to Goodyear, which had been a subcontractor for production of Black Widow components. In addition, a minimally-modified version of the Navy’s F7F Tigercat fighter was adopted as the USAF standard night fighter being designated the P-65A.

It was not until early 1945 that the first production P-61C-1-NO rolled off the production lines. The performance was substantially improved in spite of a two-thousand pound increase in empty weight. Maximum speed was 430 mph at 30,000 feet, service ceiling was 41,000 feet, and an altitude of 30,000 feet could be attained in 14.6 minutes. For added fuel capacity, the P-61C was equipped with four underwing pylons (two inboard of the nacelles, two outboard) which could carry four 310-gallon drop tanks. Most pilots who flew the P-61C felt that the increase in weight made the aircraft a lot less maneuverable than the A or B versions. The service life of the P-61C was quite brief, since it was already being replaced by the higher-performance P-65.

Northrop P-61D Black Widow

Although technically listed as a Northrop aircraft, the P-61D was in fact entirely a Goodyear development. With the night fighter role being taken over by the P-65, the Black Widow was being increasingly used for night intruder operations, for which its heavy load carrying ability and docile handling made it eminently suitable. The P-61D was optimized for the low-level night intruder role, reverting to the unturbocharged 2250 hp R-2800-65 Double Wasps of the P-61B. Goodyear had come to the conclusion that the Black Widow was seriously over-engineered and that the design could be simplified with consequent reductions in weight and production time. The four 20mm guns in the belly were replaced by Russian V-YA 23mm cannon, resulting in significant improvement in reliability and a great improvement in armor piercing capability. Underwing weapons stowage was increased by the addition of launch stubs for 12 5 inch rockets in addition to a maximum bomb load of 6,400 pounds. P-61Ds entered service in the fall of 1945 with the last being delivered in mid-1946. Production was terminated so that Goodyear could concentrate on F2G Super-Corsair production, the night intruder groups receiving P-65s in place of the Black Widows.

DP-61A/B

With most of the early P-61As and Bs being replaced by P-65s or P-61Cs, the aircraft became available for other roles. Many were brought back to the US, stripped of their armament and used as air-sea rescue planes. In this role, the proved of great value in the final hours of The Big One, meeting damaged or lost B-36s over the Atlantic and escorting them home.

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